SatyaCodes

Java- Phaser

5.Phaser

Phaser is like a collection of advantages of CountDownLatch and CyclicBarrierClasses

The CountDownLatch is :

  • Created with a fixed number of threads
    final CountDownLatch latch = new CountDownLatch(3);

  • Cannot be reset

  • Allows threads to wait (await()) or continue with its execution once count becomes 0 countDown()

the CyclicBarrier :

  • Can be reset.

  • Does not a provide a method for the threads to advance. The threads have to wait till all the threads arrive.

  • Created with fixed number of threads.

Now, the Phaser has following properties

  • Number of threads need not be known at Phaser creation time. Threads can be added dynamically.

  • Can be reset and hence is, reusable.

  • Allows threads to wait(Phaser#arriveAndAwaitAdvance()) or continue with its execution(Phaser#arrive()).

  • Supports multiple Phases(, hence the name phaser).

We will try to understand how the Phaser Class can be used with an example. In this example, we are creating a three threads, which will wait for the arrival all the threads being created.

Once all the threads have arrived(marked by arriveAndAwaitAdvance() method) the Phaser allows them through the barrier.

Methods

  • awaitAdvance()

  • arrive()

Scenario

public class PhaserExample {
	public static void main(String[] args) throws InterruptedException {
 Phaser phaser = new Phaser();
 phaser.register();// register self... phaser waiting for 1 party (thread)
 int phasecount = phaser.getPhase();
 System.out.println("Phasecount is " + phasecount);
 new PhaserExample().testPhaser(phaser, 2000);// phaser waiting for 2 parties
 new PhaserExample().testPhaser(phaser, 4000);// phaser waiting for 3 parties
 new PhaserExample().testPhaser(phaser, 6000);// phaser waiting for 4 parties
 // now that all threads are initiated, we will de-register main thread
 // so that the barrier condition of 3 thread arrival is meet.
 phaser.arriveAndDeregister();
 Thread.sleep(10000);
 phasecount = phaser.getPhase();
 System.out.println("Phasecount is " + phasecount);

	}

	private void testPhaser(final Phaser phaser, final int sleepTime) {
 phaser.register();
 new Thread() {
 	@Override
 	public void run() {
  try {
  System.out.println(Thread.currentThread().getName() + " arrived");
  phaser.arriveAndAwaitAdvance();// threads register arrival to the phaser.
  	Thread.sleep(sleepTime);
  }

  catch (InterruptedException e) {
  	e.printStackTrace();
  }
 	System.out.println(Thread.currentThread().getName() + " after passing barrier");
 	}
 }.start();
	}
}
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